The present invention relates to an oxide superconducting wire and a production method therefor, and more particularly, relates to an oxide superconducting wire having an insulating layer and a production method therefor.
Conventionally, a bismuth-based oxide superconducting wire is known as one type of oxide superconducting wire. The bismuth-based oxide superconducting wire can be used at a liquid nitrogen temperature and can yield a relatively high critical current density. Moreover, since the bismuth-based oxide superconducting wire can be relatively easily elongated, applications thereof to superconducting cables and magnets are expected.
When such an oxide superconducting wire is applied to a magnet, it is required to have an insulating layer, from the viewpoint of coiling efficiency and the like.
The above-described bismuth-based oxide superconducting wire is used as a tape-shaped wire in many cases for the following reasons. That is, the critical current density of an oxide superconductor exhibits considerably high anisotropy, and therefore, it is necessary to align polycrystals of the oxide superconductor in order to achieve a high critical current density.
The bismuth-based oxide superconducting wire is deformed into a tape by plastic deformation such as rolling. The polycrystals of precursor of the oxide superconductor are aligned by the plastic deformation.
The width of the tape-shaped oxide superconducting wire varies within the range of xc2x10.2 mm in the above-described plastic deformation process. It is therefore necessary to adopt a method for reliably forming an insulating layer over the entire surface of the tape-shaped wire even when the width of the wire varies. As a result, it has been considered to adopt a method of forming an insulating layer in the above tape-shaped wire in which an insulating-film base is applied to a tape-shaped wire by putting the tape-shaped wire between felts impregnated with the insulating-film base and the tape-shaped wire is then subjected to baking. The tape-shaped wire in such an insulating layer forming method using felts is coated solely with a layer of the insulating-film base of approximately 1.5 xcexcm in thickness in one operation of applying the insulating-film base. For this reason, in such an insulating layer forming method using the felts, an operation of applying the insulating-film base and a baking operation are usually repeated about ten times.
However, when the above insulating layer forming method is applied to a tape-shaped bismuth-based oxide superconducting wire, the critical current density of the tape-shaped oxide superconducting wire decreases substantially after an insulating layer is formed. This is due to the following reasons.
When the above insulating layer forming method is applied to a tape-shaped oxide superconducting wire, the tape-shaped wire is heated at a high temperature in the baking process. The temperature of the tape-shaped wire is raised by the high-temperature heating. The tape-shaped wire is composed of oxide superconducting filaments and a metal covering layer formed around the oxide superconducting filaments. With the above-described increase in temperature of the tape-shaped wire, the metal covering layer and the oxide superconducting filaments in the tape-shaped wire are thermally expanded. In this case, strain occurs in the tape-shaped wire due to a difference in coefficient of thermal expansion between the metal covering layer and the oxide superconducting filaments. For this reason, a mechanical strain is exerted on the oxide superconducting filaments. As a result, superconducting properties are deteriorated, for example, the critical current density of the tape-shaped oxide superconducting wire is lowered.
When the insulating-film base applying and the baking are repeated about ten times, it is necessary to put the tape-shaped wire into a baking furnace a predetermined number of times. In this case, since the tape-shaped wire is led into the baking furnace a plurality of times, the direction of travel of the tape-shaped wire occasionally changes by using a roller, although this depends on the configuration of an apparatus for applying and baking the insulating-film base. Since the tape-shaped wire is bent along the roller, it is subjected to bending.
In order to move the tape-shaped wire in the felts and the baking furnace, it is necessary to constantly apply a fixed tension to the tape-shaped wire. Such bending of the tape-shaped wire and application of tension thereto also put excessive mechanical strain on the oxide superconducting filaments. As a result, the superconducting properties of the tape-shaped oxide superconducting wire are deteriorated, and the critical current density is lowered.
As described above, it is difficult to form an insulating layer in the conventional tape-shaped oxide superconducting wire without deteriorating the superconducting properties.
The present invention has been developed to solve the above problems, and an object of the invention is to provide an oxide superconducting wire which allows an insulating layer to be formed without deteriorating the superconducting properties, and to provide a production method therefor.
According to a first aspect of the present invention, an oxide superconducting wire includes an oxide superconducting filament, a matrix, a covering layer, and an insulating layer. The matrix is made of silver and is placed so as to enclose the oxide superconducting filament. The covering layer, placed so as to enclose the matrix, contains silver and manganese, and has a thickness of 10 xcexcm to 50 xcexcm. The insulating layer is placed so as to enclose the covering layer.
Since the covering layer is made of a material containing silver and manganese, the mechanical strength thereof can be increased. This ensures sufficient strength to withstand the tension and bending applied to the oxide superconducting wire when forming the insulating layer. That is, since the covering layer of the oxide superconducting wire thus has sufficient strength, it is possible to prevent the exertion of excessive mechanical strain on the oxide superconducting filament in the process of forming the insulating layer, the process of constructing a magnet, a cable, or the like by using the oxide superconducting wire, and in the case in which stress is applied to the oxide superconducting wire by a temperature change due to cooling and electromagnetic force when operating an equipment using the oxide superconducting wire, such as a magnet. As a result, it is possible to prevent deterioration of the superconducting properties of the oxide superconducting filament. For this reason, the critical current density of the oxide superconducting wire is prevented from being lowered.
While manganese is used as an element to be contained in the covering layer in order to increase the strength thereof, it has a relatively low reactivity with an oxide superconductor. For this reason, it is possible to inhibit the problem that the element in the covering layer in a sintering process for generating the oxide superconductor hinders the generative reaction of an oxide superconductor. Since the matrix of silver is interposed between the oxide superconducting filament and the covering layer, manganese in the covering layer can be reduced by the matrix from diffusing to the oxide superconducting filament. This more reliably reduces the above problem in which the generative reaction of the oxide superconductor is hindered by manganese in the covering layer.
Since the thickness of the covering layer is within the range of 10 xcexcm to 50 xcexcm, the covering layer can be formed without causing fatal defects, such as cracks, in the production procedure for the oxide superconducting wire. Moreover, the oxide superconductor can be generated reliably because gas generated with the generative reaction of the oxide superconductor can be reliably released from the wire. In the case in which the thickness of the covering layer is less than 10 xcexcm, defects, such as cracks, arise in the covering layer in a step of shaping the oxide superconducting wire in the production procedure therefor. In the case in which the thickness of the covering layer is more than 50 xcexcm, gas generated with the generative reaction of the oxide superconductor is not properly released from the wire, which results in an incomplete generative reaction of the oxide superconductor. Alternatively, a phenomenon sometimes occurs in which a cavity is formed in the wire due to the gas. In this case, the superconducting properties of the oxide superconducting wire are markedly deteriorated. Furthermore, in the case in which the thickness of the covering layer is more than 50 xcexcm, the ratio of the occupied area of the oxide superconducting filament to the total cross-sectional area of the oxide superconducting wire decreases, and therefore, the current density of the oxide superconducting wire is lowered.
In the case in which the oxide superconducting wire of the present invention is applied to equipment, such as a magnet, superior coiling efficiency can be obtained because of the insulating layer of the oxide superconducting wire. This makes it possible to improve the packing rate and the dimensional accuracy of the oxide superconducting wire and to easily obtain a high insulating property, which differs from the conventional coiling operation in which insulating materials are put between the portions of the superconducting wire.
Since the insulating layer is formed beforehand, when the oxide superconducting wire is exposed to a cryogen, such as liquid nitrogen or liquid helium, the insulating layer can be used as a protective film for preventing the cryogen from entering the interior of the covering layer or the matrix. In the case in which an oxide superconducting wire having no insulating layer is applied to a magnet, a cable, and the like, it is necessary to insulate portions of the oxide superconducting wire by some means. For example, a means is considered in which a coiling operation is performed while putting an insulating material between the portions of the oxide superconducting wire. In the case in which a cryogen is used to cool an equipment thus produced, such as a magnet, it sometimes directly contacts a portion of the surface of an oxide superconducting wire constituting the device, where an insulating layer is not formed. If the surface of the oxide superconducting wire has a defect, such as a pinhole, the cryogen enters the interior of the oxide superconducting wire. When the temperature of the oxide superconducting wire rises above the boiling point of the cryogen in this state, the cryogen is vaporized inside the wire. The vaporized cryogen forms a cavity inside the wire. When such a cavity is formed, the superconducting properties of the oxide superconducting wire having no insulating layer are markedly deteriorated. Since the oxide superconducting wire of the present invention has the insulating layer, however, the above problem is avoided.
It is preferable that the covering layer in the oxide superconducting wire according to the above first aspect contain 0.1% to 0.5% manganese by weight.
This makes it possible to reliably increase the strength of the covering layer and to prevent manganese from hindering a generative reaction of the oxide superconductor. When the manganese content is less than 0.1% by weight, the strength of the covering layer cannot be sufficiently increased. When the manganese content is in excess of 0.5% by weight, in the oxide superconducting wire using the silver matrix according to the first aspect, the manganese in the covering layer diffuses and reaches the interior of the matrix. As a result, a generative reaction of the oxide superconductor is hindered by manganese.
According to another aspect of the present invention, an oxide superconducting wire includes an oxide superconducting filament, a matrix, a covering layer, and an insulating layer. The matrix contains silver and antimony and is placed so as to enclose the oxide superconducting filament. The covering layer is placed so as to enclose the matrix, contains silver and manganese, and has a thickness of 10 xcexcm to 50 xcexcm. The insulating layer is placed so as to enclose the covering layer.
Since a material containing silver and manganese is used as the covering layer, in a manner similar to that of the oxide superconducting wire according to the above first aspect of the present invention, the mechanical strength of the covering layer can be increased. This makes it possible to ensure strength of the covering layer that sufficiently withstands tension and bending to be applied to the oxide superconducting wire when forming the insulating layer. That is, it is possible to prevent the exertion of excessive mechanical strain on the oxide superconducting filament in the process of forming the insulating layer or the like. As a result, it is possible to prevent the deterioration of the superconducting properties of the oxide superconducting filament. Consequently, this prevents the decrease of the critical current density of the oxide superconducting wire.
While manganese is used as an element to be contained in the covering layer in order to increase strength, it has a relatively low reactivity with an oxide superconductor. For this reason, it is possible to inhibit the problem that the element in the covering layer in a sintering process for generating the oxide superconductor hinders the generative reaction of an oxide superconductor. Since the matrix containing silver and antimony is interposed between the oxide superconducting filament and the covering layer, manganese in the covering layer can be restricted from diffusing to the oxide superconducting filament by the matrix. As a result, it is possible to more reliably avoid the above problem, that is, the hindrance of the generative reaction of the oxide superconductor by manganese in the covering layer.
The matrix contains antimony. Antimony serves to restrict manganese in the covering layer from diffusing to a region where the superconducting filament is placed. For this reason, the manganese content of the covering layer can be made higher than that in the case in which only silver is used as the matrix, without deteriorating the superconducting properties of the oxide superconducting wire. This further increases the mechanical strength of the covering layer.
Since the thickness of the covering layer is 10 xcexcm to 50 xcexcm, it is possible to form the covering layer in a production procedure, which will be described later, without causing fatal defects such as cracks, in a manner similar to that of the oxide superconducting wire according to the above first aspect. Moreover, it is possible to reliably release gas, which is generated with the generative reaction of the oxide superconductor, from the wire. For this reason, the oxide superconductor can be formed reliably. When the thickness of the covering layer is less than 10 xcexcm, defects, such as cracks, occur in the covering layer in the wire shaping process of the production procedure for the oxide superconducting wire. When the thickness of the covering layer is more than 50 xcexcm, gas, which is generated with the generative reaction of the oxide superconductor, is not sufficiently released from the wire, and this results in an incomplete generative reaction of the oxide superconductor. Inside the wire, a cavity is sometimes formed due to the above-described gas. This markedly deteriorates the superconducting properties of the oxide superconducting wire. When the thickness of the covering layer is more than 50 xcexcm, the ratio of the occupied area of the oxide superconducting filament to the total cross-sectional area of the oxide superconducting wire is decreased, and therefore, the current density of the oxide superconducting wire is markedly lowered.
Since the oxide superconducting wire includes the insulating layer, in the case in which the oxide superconducting wire of the present invention is applied to equipment, such as a magnet, superior coiling efficiency can be obtained. In contrast to the conventional case in which insulating materials are put between the portions of the superconducting wire during a coiling operation, it is also possible to improve the packing rate and the dimensional accuracy of the oxide superconducting wire and to easily obtain a high insulating property.
Since the insulating layer is formed beforehand, when the oxide superconducting wire is exposed to a cryogen, such as liquid nitrogen or liquid helium, the insulating layer can be used as a protective film for preventing the cryogen from entering the interior of the covering layer or the matrix.
In the above-described oxide superconducting wire according to another aspect, it is preferable that the matrix contain 0.1% to 0.5% antimony by weight, and that the covering layer contain 0.5% to 1.0% manganese by weight.
In this case, the mechanical strength of the covering layer can be sufficiently increased by manganese, and manganese in the covering layer can be more reliably prevented from diffusing into the matrix by antimony. That is, since the diffusion of manganese can be inhibited by antimony, the manganese content of the covering layer can be made higher than that in the above-described oxide superconducting wire according to the first aspect. This can increase the mechanical strength of the covering layer.
When the antimony content of the matrix exceeds 0.5% by weight, the superconducting properties of the oxide superconducting filament are deteriorated. For this reason, the critical current density of the oxide superconducting wire is lowered. When the antimony content of the matrix is less than 0.1% by weight, it is impossible to sufficiently obtain the antimony""s effect of preventing the diffusion of manganese.
When the manganese content of the covering layer is more than 0.5% by weight, it is possible to obtain sufficient strength of the covering layer which is greater than that in the oxide superconducting wire according to the first aspect of the present invention. In contrast, when the manganese content exceeds 1.0% by weight, manganese sometimes diffuses and reaches a region where the oxide superconducting filament is placed. In this case, a generative reaction of an oxide superconductor is hindered by manganese in the production procedure for the oxide superconducting wire.
In the oxide superconducting wire according to the first aspect or another aspect, it is preferable that manganese be dispersed as oxide particles in the covering layer (claim 5).
In this case, since the oxide particles of manganese are dispersed in the covering layer, they can reliably increase the mechanical strength of the covering layer.
In the oxide superconducting wire according to the first aspect or another aspect, it is preferable that manganese exists while being in a solid solution in the covering layer.
In this case, lattice strain is formed in a material other than manganese (e.g., silver), which constitutes the covering layer, due to manganese being in a solid solution in the material. This can increase the mechanical strength of the covering layer.
In the oxide superconducting wire according to the first aspect or another aspect, it is preferable that the thickness of the covering layer be within the range of 20 xcexcm to 40 xcexcm.
In this case, it is possible to reliably achieve a high critical current density of the oxide superconducting wire and to reliably release gas, which results from a generative reaction of the oxide superconductor, from the wire. For this reason, it is possible to obtain an oxide superconducting wire having superior superconducting properties.
According to the first aspect or another aspect of the oxide superconducting wire, it is preferable that the oxide superconducting wire be shaped like a tape having a flat portion.
This allows a coiling operation to be easily performed when the oxide superconducting wire of the present invention is applied to a magnet or the like.
When forming an insulating layer in the tape-shaped oxide superconducting wire, a method is adopted to form an insulating layer by using felt. In the case in which the operation of applying a base to become an insulating layer and a baking operation are repeated a plurality of times, as in such insulating layer forming method by using a felt, the superconducting properties can be reliably prevented from being deteriorated in the oxide superconducting wire of the present invention. For this reason, the above-described advantages of the present invention are prominent, in particular, in the tape-shaped oxide superconducting wire.
According to the first aspect or another aspect of the oxide superconducting wire, it is preferable that the thickness of the covering layer at the flat portion be within the range of 10 xcexcm to 50 xcexcm.
In this case, since the flat portion constitutes a large proportion of the surface of the tape-shaped oxide superconducting wire, when the thickness of the covering layer at the flat portion is within the range of 10 xcexcm to 50 xcexcm, as described above, the above-described advantages of the oxide superconducting wire of the present invention can be provided more reliably.
It is preferable that in the oxide superconducting wire according to the first aspect or another aspect the thickness of the covering layer at the flat portion be within the range of 20 xcexcm to 40 xcexcm.
In this case, it is possible to reliably achieve a high critical current density in the oxide superconducting wire, as described above, and to reliably release gas, which is generated due to a generative reaction of an oxide superconductor, from the wire. This makes it possible to more reliably obtain an oxide superconducting wire having superior superconducting properties.
It is preferable that in the oxide superconducting wire according to the first aspect or another aspect the thickness of the insulating layer be within the range of 5 xcexcm to 100 xcexcm.
When the thickness of the insulating layer is less than 5 xcexcm, a local dielectric breakdown is prone to occur, and defects, such as pinholes, are prone to occur in the insulating layer. When equipment using the oxide superconducting wire is operating, the oxide superconducting wire is cooled by a cryogen such as liquid nitrogen. In this case, the cryogen sometimes enters the interior of the insulating layer from the defects. If the temperature of the oxide superconducting wire rises above the boiling point of the cryogen in this state, the cryogen vaporizes. For this reason, the insulating layer is broken by the vaporized cryogen, or a cavity is formed inside the oxide superconducting wire and the oxide superconducting wire becomes a partially bulged shape. In this case, the superconducting properties of the oxide superconducting wire are substantially deteriorated. When the thickness of the insulating layer exceeds 100 xcexcm, the ratio of the occupied area of the superconducting filament to the cross-sectional area of the oxide superconducting wire decreases, and therefore, the critical density decreases. When the thickness of the insulating layer is within the above range, the above problems can be avoided.
It is preferable that in the oxide superconducting wire according to the first aspect or another aspect the insulating layer contains resin.
In this case, since resin can be baked in a temperature range that does not deteriorate the properties of the oxide superconducting wire, an insulating layer can be formed without deteriorating the superconducting properties of the oxide superconducting filament. The use of resin also permits an insulating layer with a relatively small thickness having no defects such as pinholes. As a result, it is possible to achieve a high current density and to effectively prevent the penetration of the cryogen into the interior of the oxide superconducting wire. By selecting an appropriate resin, it is possible to achieve an insulating layer having sufficient insulating properties, water resistance, stability with respect to temperature change, and heat resistance.
It is preferable that in the oxide superconducting wire according to the first aspect or another aspect the resin be a formal resin.
A formal resin can be baked at a baking temperature of 400xc2x0 C. or less which is relatively lower than that for other resins. Therefore, the baking temperature in a process for forming the insulating layer can be made lower than that in cases where other resins are used. This decreases the temperature for heating the oxide superconducting wire in the process for forming the insulating layer. For this reason, it is possible to reduce mechanical strain due to the heat in the oxide superconducting filament. As a result, it is possible to reliably prevent the deterioration of the superconducting properties of the oxide superconducting wire.
It is preferable that in the oxide superconducting wire according to the first aspect or another aspect the oxide superconducting filament be a bismuth-based oxide superconducting filament.
A bismuth-based oxide superconductor can be used at liquid nitrogen temperature. Since the bismuth-based oxide superconductor can yield a relatively high critical current density and can be elongated, the application thereof to a magnet or the like is expected. In the application to a magnet or the like, an oxide superconducting wire having an insulating layer is required from the viewpoint of coiling efficiency and the like. For this reason, the application of the present invention to the bismuth-based oxide superconducting wire provides particularly noticeable advantages.
An oxide superconducting wire production method according to a further aspect of the present invention includes a step of preparing a wire having powder of precursor to be sintered into an oxide superconductor, a matrix made of silver and placed so as to enclose the powder of precursor, and a covering layer containing silver and 0.1% to 0.5% manganese by weight placed so as to enclose the matrix; a sintering step of forming an oxide superconductor from the powder of precursor by heating the wire; and a coating step for forming an insulating layer on the outer surface of the covering layer so as to enclose the covering layer in a state in which tension is applied to the wire having the oxide superconductor in the longitudinal direction.
Since the covering layer thus contains silver and 0.1% to 0.5% manganese by weight, it is possible to obtain a wire having adequate strength to sufficiently withstand tension and bending to be applied to the wire in the covering step of forming the insulating layer. That is, since the covering layer of the oxide superconducting wire thus has sufficient strength, it is possible to prevent the exertion of excess mechanical strain onto the oxide superconductor in the coating step of forming the insulating layer. As a result, it is possible to prevent the decrease of the critical current density of the oxide superconducting wire due to excessive mechanical strain.
When the manganese content is less than 0.1% by weight, it is impossible to sufficiently increase the strength of the covering layer. When the manganese content is more than 0.5% by weight, in the wire having the matrix made of silver, the manganese in the covering layer diffuses and reaches the interior of the matrix in the sintering step. As a result, the generative reaction of the oxide superconductor in the sintering step is hindered by manganese.
An oxide superconducting wire production method according to a still further aspect of the present invention includes a step of preparing a wire having powder of precursor to be sintered into an oxide superconductor, a matrix containing silver and 0.1% to 0.5% antimony by weight placed so as to enclose the powder of precursor, and a covering layer containing silver and 0.5% to 1.0% manganese by weight placed so as to enclose the matrix; a sintering step of forming an oxide superconductor from the powder of precursor by heating the wire; and a coating step of forming an insulating layer on the outer surface of the covering layer so as to enclose the covering layer in a state in which tension is applied to the wire having the oxide superconductor in the longitudinal direction.
Since a material containing silver and 0.5% to 1.0% manganese by weight is used as the covering layer, the mechanical strength of the covering layer can be further increased, compared with that in the oxide superconducting wire production method according to the above further aspect. This makes it possible to ensure strength of the covering layer that sufficiently withstands tension and bending to be applied to the wire in the coating step. That is, it is possible to prevent the exertion of excessive mechanical strain on the oxide superconductor in the coating step of forming the insulating layer. As a result, it is possible to prevent the decrease of the critical current density of the oxide superconducting wire.
Manganese in the covering layer can be reliably prevented by antimony in the matrix from diffusing into the matrix.
When the antimony content of the matrix exceeds 0.5% by weight, the superconducting properties of the oxide superconducting filament are deteriorated. When the antimony content of the matrix is less than 0.1% by weight, it is impossible to sufficiently obtain the effect of the antimony described above which prevents the diffusion of manganese.
When the manganese content of the covering layer is 0.5% or more by weight, it is possible to obtain a sufficient mechanical strength of the covering layer which is higher than that of the covering layer according to the above further aspect. In contrast, when the manganese content exceeds 1.0% by weight, manganese sometimes diffuses and reaches the region of the oxide superconductor. In this case, a generative reaction of the oxide superconductor is hindered by manganese in the sintering step.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the preparation step include a step of filling a first pipelike member to become the matrix with powder of precursor, a step of drawing the first pipelike member filled with the powder of precursor, a step of placing the narrowed first pipelike member inside a second pipelike member to become a covering layer, and a step of drawing the second pipelike member with the narrowed first pipelike member placed therein.
In this case, the preparation step can be carried out more easily than a case in which a sheetlike member is used as a member to become a covering layer.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the thickness of the covering layer in the preparation step be within the range of 10 xcexcm to 50 xcexcm.
In this case, since gas, which is generated with a generative reaction of the oxide superconductor in the sintering process, can easily pass through the covering layer, it is reliably released from the wire. For this reason, an oxide superconductor can be generated reliably. When the thickness of the covering layer is less than 10 xcexcm, defects, such as cracks, sometimes occur in the covering layer in the preparation step. When the thickness of the covering layer is more than 50 xcexcm, gas generated with the generative reaction of the oxide superconductor is not properly released from the wire, which results in an incomplete generative reaction of the oxide superconductor. A cavity is sometimes formed in the wire due to the gas. In this case, the superconducting properties of the produced oxide superconducting wire are markedly deteriorated.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the thickness of the covering layer in the preparation step be within the range of 20 xcexcm to 40 xcexcm.
In this case, it is possible to prevent defects from occurring in the covering layer and to reliably release gas, which is generated due to a generative reaction of an oxide superconductor in the sintering step, from the wire.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the wire be shaped like a tape having a flat portion in the preparation step.
In this case, since the surface area of the covering layer is made larger than that of a wire with a circular cross section, it is possible to more reliably release gas, which is generated due to a generative reaction of an oxide superconductor in the sintering step, from the wire via the covering layer.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the thickness of the covering layer at the flat portion be within the range of 10 xcexcm to 50 xcexcm.
In this case, since the flat portion constitutes a large proportion of the surface of the oxide superconducting wire, when the thickness of the covering layer at the flat portion is within the range of 10 xcexcm to 50 xcexcm, as described above, it is possible to more reliably release gas, which is generated due to a generative reaction of an oxide superconductor in the sintering step, from the wire. This reliably yields an oxide superconducting wire having superior superconducting properties.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the thickness of the covering layer at the flat portion be within the range of 20 xcexcm to 40 xcexcm.
In this case, it is possible to reliably prevent defects from occurring in the covering layer and to reliably release gas, which is generated due to a generative reaction of an oxide superconductor in the sintering step, from the wire.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the thickness of the insulating layer in the coating step be within the range of 5 xcexcm to 100 xcexcm.
When the thickness of the insulating layer is less than 5 xcexcm, defects, such as pinholes, are prone to occur in the insulating layer in the coating step. When the thickness of the insulating layer exceeds 100 xcexcm, the ratio of the occupied area of the oxide superconductor to the cross-sectional area of the oxide superconducting wire decreases, and therefore, the current density decreases.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the insulating layer contains resin.
In this case, since resin can be baked in a temperature range that does not deteriorate the properties of the oxide superconducting wire, the coating step can be carried out without deteriorating the superconducting properties of the oxide superconducting wire.
In the oxide superconducting wire production method according to the further or still further aspect, it is more preferable that the resin be a formal resin.
A formal resin can be baked at a temperature of 400xc2x0 C. or less which is relatively lower than those for other resins. Therefore, the baking temperature in the coating step can be lowered, compared with those in cases in which other resins are used.
In the oxide superconducting wire production method according to the further or still further aspect, it is preferable that the oxide superconductor be a bismuth-based oxide superconductor.
A bismuth-based oxide superconductor can be used at the liquid nitrogen temperature, can yield a relatively high critical current density, and can be elongated. Therefore, an application thereof to an equipment, such as a magnet, is expected. From the viewpoint of coiling efficiency when producing such equipment, an oxide superconducting wire having an insulating layer is required. For this reason, application of the present invention to a production method for a bismuth-based oxide superconducting wire provides particularly noticeable advantages.
An oxide superconducting wire according to a still further aspect of the present invention includes an oxide superconducting filament, a covering layer, and an insulating layer. The covering layer is placed so as to enclose the oxide superconducting filament and includes a silver alloy containing 0.1% to less than 1.0% manganese by weight. The insulating layer is placed so as to enclose the covering layer.
Since the oxide superconducting wire has the insulating layer, superior coiling efficiency can be obtained when the oxide superconducting wire of the present invention is applied to a magnet or the like.
Since the covering layer includes a silver alloy containing 0.1% to less than 1.0% manganese by weight, it is possible to ensure a strength of the oxide superconducting wire that sufficiently withstands tension and bending to be applied to the oxide superconducting wire when forming the insulating layer. That is, since the covering layer of the oxide superconducting wire has such sufficient strength, it is possible to prevent the exertion of excessive mechanical strain on the oxide superconducting filament in the step of forming the insulating layer. As a result, it is possible to prevent the deterioration of the superconducting properties of the oxide superconducting filament. This can prevent the critical current density of the oxide superconducting wire from being lowered.
When the manganese content is 1.0% or more by weight, manganese sometimes enters the core portion of the oxide superconducting wire and reaches the oxide superconducting filament. In such a case, the critical current density of the oxide superconducting wire is lowered due to the entry of manganese. When the manganese content is 0.1% or more by weight, it is possible to reliably increase the tensile strength of the second covering layer. As a result, the tensile strength of the oxide superconducting wire can be reliably increased.
In the oxide superconducting wire according to the still further aspect, it is preferable that the manganese content of the silver alloy in the covering layer be within the range of 0.5% to less than 1.0% by weight.
In this case, it is possible to reliably increase the tensile strength of the oxide superconducting wire in a high-temperature environment. As a result, it is possible to more reliably prevent the deterioration of the superconducting properties of the oxide superconducting wire in the step of forming the insulating layer.
In the oxide superconducting wire according to the still further aspect, it is preferable that the covering layer includes a first covering layer and a second covering layer.
In the oxide superconducting wire according to the still further aspect, it is preferable that the first covering layer contains silver.
In the oxide superconducting wire according to the still further aspect, it is preferable that the first covering layer be made of a silver alloy containing antimony and that the second covering layer be placed so as to enclose the first covering layer and be made of a silver alloy containing 0.1% to less than 1.0% manganese by weight.
In this case, manganese contained in the second covering layer can be inhibited from diffusing and entering the interior of the oxide superconducting filament by the antimony contained in the first covering layer. As a result, it is possible to more reliably prevent the deterioration of the superconducting properties of the oxide superconducting wire due to the diffusion of manganese.
In the oxide superconducting wire according to the still further aspect, it is preferable that the first covering layer be made of a silver alloy containing 0.1% to less than 0.5% antimony by weight.
In this case, when the antimony content is within the above range, antimony is inhibited from diffusing to the oxide superconducting filament. Moreover, the diffusion of manganese to the oxide superconducting filament can be prevented by antimony in the first covering layer. As a result, it is possible to more reliably prevent the deterioration of the superconducting properties of the oxide superconducting wire.
When the antimony content is 0.5% or more by weight, the superconducting properties of the oxide superconducting filament are deteriorated. For this reason, the critical current density of the oxide superconducting wire is lowered.
Since the antimony content is 0.1% or more by weight, the above effect of preventing diffusion of manganese can be reliably achieved.
Since the silver alloy having the above antimony content is used as the first covering layer, it is possible to further increase the tensile strength of the oxide superconducting wire. For this reason, it is possible to more reliably prevent the exertion of excessive mechanical strain on the oxide superconducting filament in the step of forming the insulating layer in the oxide superconducting wire. This can reliably prevent the deterioration of the superconducting properties of the oxide superconducting wire.
In the oxide superconducting wire according to the still further aspect, it is preferable that the thickness of the insulating layer be within the range of 10 xcexcm to 100 xcexcm.
In the production procedure of the oxide superconducting wire, the oxide superconducting wire is sintered at a high temperature of 800xc2x0 C. or more and in an oxidizing atmosphere. For this reason, coarsening of crystal grains, oxidation and precipitation of an additional element, and the like sometimes occur in the second covering layer. For example, a protuberance of approximately 2 xcexcm to 10 xcexcm is sometimes formed on the surface of the second covering layer including the silver alloy containing manganese by the above sintering process. For this reason, when the above protuberance is formed in the sintering process, the occurrence of defects, such as pinholes, can be prevented in the insulating layer by the protuberance, by setting the thickness of the insulating layer at 10 xcexcm or more. This can reliably insulate the oxide superconducting wire.
When the thickness of the insulating layer exceeds 100 xcexcm, the ratio of the oxide superconducting filament to the entire oxide superconducting wire decreases. This makes it difficult to obtain predetermined electric characteristics, such as the amount of current per unit sectional area, in the oxide superconducting wire.
In the oxide superconducting wire according to the still further aspect, it is preferable that the insulating layer contains a formal resin.
A formal resin can be baked at a temperature of 400xc2x0 C. or less which is relatively lower than those for other resins. Therefore, the baking temperature in the step of forming the insulating layer can be made lower than those in cases in which other resins are used. This decreases the temperature for heating the oxide superconducting wire in the step of forming the insulating layer. For this reason, it is possible to reduce mechanical strain due to the heat in the oxide superconducting filament. As a result, it is possible to reliably prevent the deterioration of the superconducting properties of the oxide superconducting wire.
In the oxide superconducting wire according to the still further aspect, it is preferable that the oxide superconducting wire be shaped like a tape in outline.
In this case, it is possible to easily perform a coiling operation or the like when applying the oxide superconducting wire of the present invention to a magnet or the like.
In order to form an insulating layer in a tape-shaped oxide superconducting wire, a method is adopted to form an insulating layer by using a felt. In a case in which an operation of applying a base to produce the insulating layer and a baking operation are repeated a plurality of times, such as in the insulating layer forming method using the felt, the oxide superconducting wire of the present invention can reliably prevent the superconducting properties from deterioration. For this reason, the above advantages of the present invention are particularly noticeable in the tape-shaped oxide superconducting wire.
In the oxide superconducting wire according to the still further aspect, it is preferable that the oxide superconducting filament be a bismuth-based oxide superconducting filament.
In this case, a bismuth-based oxide superconductor can be used at the liquid nitrogen temperature. Since the bismuth-based oxide superconductor can yield a relatively high critical current density and can be elongated, an application thereof to a magnet or the like is expected. In the application to a magnet or the like, an oxide superconducting wire having an insulating layer is required from the viewpoint of coil efficiency and the like. For this reason, particularly noticeable advantages can be provided by applying the present invention to the bismuth-based oxide superconducting wire.